1. Field of the Invention
The present invention relates to an information recorder such as an optical disk unit, a magnetooptic disk unit, etc. for irradiating a light spot to a rotating disk-shaped recording medium and recording and regenerating information with respect to tracks on the recording medium. More particularly, the present invention relates to an information recorder of this kind for controlling a seeking operation in which the light spot is moved from one track to another track.
2. Description of the Related Art
For example, Japanese Patent Application Laying Open (KOKAI) No. 2-33731 shows a device for performing the seeking operation of this kind. This device as a general example relates to an optical disk unit. In this device, a track pulse is generated every time a light spot crosses a track on an optical disk. The number of track pulses is counted by an updown counter preset to a small value. The optical disk unit makes a reference speed signal proportional to this counting value as an absolute value. A moving speed of the light spot is detected from the frequency of a track pulse. An actuator for slightly moving an objective lens (i.e., the light spot) is driven by a difference signal between a signal indicative of this moving speed and the reference speed signal. Thus, the objective lens (i.e., the light spot) is relatively displaced with respect to an optical head. A seek motor is driven by a signal proportional to an amount of this displacement. Accordingly, the optical head is moved in accordance with the movement of the objective lens (i.e., the light spot). When the counting value of the updown counter is equal to a set value, counting directions of this counter are switched. When the counting value of the updown counter is equal to -1, this counter is reset and the seeking operation is completely performed.
In the general example, the moving speed of the light spot with respect to a track is controlled such that this moving speed is in conformity with a reference or target speed. However, when the entire disk unit is inclined, no error between the moving and reference speeds can be sufficiently reduced to a small value. This problem will next be described with reference to FIG. 8.
In FIG. 8, reference numerals 401 and 402 respectively designate an optical disk and an objective lens. An actuator 404 slightly displaces the objective lens 402 with respect to an optical head in a direction perpendicular to a track. Namely, the actuator 404 slightly displaces a light spot converged by this objective lens 402 with respect to the optical head in a direction perpendicular to a track. Reference numeral 403 designates a horizontal plane. An inclination angle of the entire disk unit is set to .theta.. The mass of a movable portion (i.e., the optical head) including the objective lens is set to m. In this case, force f=mgsin.theta. (g is a gravitational acceleration) is applied to the movable portion in a direction crossing the track.
In this case, if the movable portion is set to an inertial system, force equal to the above force f and having an opposite direction must be applied to the objective lens 402 so as to relatively move the light spot with respect to the optical disk 401 at an equal speed in the track crossing direction. However, in the above general example, such force is not applied to the objective lens 402 so that a speed error corresponding to this unapplied force is caused.
When an external vibration is applied to the disk unit, the speed error is also caused even when no entire disk unit is inclined. The optical disk 402 is supported by a spindle motor integrated with the disk unit. Accordingly, the optical disk 402 is moved together with the vibration of the disk unit. In contrast to this, the movable portion is supported by a bearing, a spring, etc. with respect to the disk unit. The movable portion tends to be held by law of inertia in a position irrespective of the vibration. Therefore, the speed error is also caused by the external vibration as mentioned above. Force ma must be applied to the movable portion so as to move the light spot at an equal speed with respect to a track when the optical disk 402 is vibrated at an acceleration a. A speed error corresponding to this force ma is caused.
When such a speed error is caused, the seeking operation tends to be unstable so that no seeking operation can be reliably performed.
It is sufficient to increase a loop band region of speed control to reduce the speed error. However, in reality, no loop band region can be greatly increased in view of influences of mechanical resonance of the movable portion, etc. Accordingly, it is necessary to take different measures for reducing the speed error.